Analog information is most commonly transmitted on optical fibers as an intensity modulated optical signal, and there are numerous signal processing applications that require such signals to be summed. One such application is in phased-array radar. In a phase-array radar system, the signals received by the individual antenna elements are phase-delayed, and then combined in a predetermined manner to electronically point the antenna beam in a particular direction.
In one known type of phased-array system, the electronic signal detected at each antenna element is amplified and then converted into an optical signal that is coupled into a plurality of fiber-optic cables. Each fiber-optic cable has a particular length, and therefore subjects an optical signal passing through it to a particular time or phase delay. In order to detect a signal arriving from a particular direction, the signals on a selected group of fiber-optic cables (one from each antenna element) are combined and summed to produce an electronic output signal. The lengths of the selected fiber-optic cables are such that a signal received at the antenna elements from the particular direction will result in fiber-optic cable signals that arrive simultaneously at the summing point.
In order to sum the optical signals on a particular group of fiber-optic cables, each optical signal is typically converted to an electrical signal, and the electrical signals are then input to an RF power combiner. Directly combining the optical signals in an optical coupler is not feasible, because it will produce coherent optical summing, and thereby cause unacceptable noise resulting from mixing products between the carrier frequencies of the signals that are being detected. To avoid the production of such mixing products, each carrier must have the same phase and frequency, a nearly impossible requirement in most systems. There is therefore a need for a simple technique for summing intensity modulated optical signals that may have different phases and/or carrier frequencies.